Glaucoma is a major public health problem, affecting about two percent of the U.S. population. Numerous forms of glaucoma result in elevated intraocular pressure (IOP) in the eye, which can cause pathological changes in the optic nerve, and both central and peripheral visual field loss. If the IOP remains high for an extended period of time, total vision loss can occur. The elevated IOP is caused by an imbalance in fluid inflows and outflows in the eye, and the principal objective of medical treatment is the lowering of intraocular pressure.
The anterior chamber of the eye contains the aqueous humor (aqueous), a clear fluid that is produced continuously by the ciliary body around the lens. The constant flow of aqueous humor though the eye's front chamber exits through two different routes. One significant aqueous outflow pathway is through the trabecular meshwork and the Schlemm's canal. The trabecular meshwork is filtering structure that extends 360° around the circumference of the eye at the “angle”—the junction between the iris, sclera and cornea. The trabecular meshwork consists of layers of collagen webs that filter the outflows that reach Schlemm's canal. The meshwork has a monolayer of trabecular cells that produce enzymes for degrading extra-cellular material that may be captured by the filtering structure. Schlemm's canal is a passageway or series of septae that extend around the circumference of the eye adjacent to the meshwork. The aqueous fluid exits Schlemm's canal through a series of collecting channels that drain from the canal into the episcleral venous system.
The second aqueous outflow route is through the region of the ciliary body and the uveoscleral plane, wherein fluid migrates outwardly directly through tissue and between muscle fibers of the ciliary body.
In a normal eye, aqueous production by the ciliary body is equal to aqueous outflows to provide an IOP that remains constant in the 15 to 21 mm Hg range. In a patient suffering from glaucoma, the resistance through the outflow system is over about 21 mm Hg. In primary open angle glaucoma (POAG), the most common form of glaucoma, the principal resistance to fluid outflow is centered about the region of trabecular meshwork that is adjacent Schlemm's canal.
Numerous therapies have been developed for treating glaucoma by decreasing intraocular pressure. Pharmacological therapies include topical ophthalmic drops and oral medications that reduce the production of aqueous by the ciliary body or increase aqueous outflows via the uveoscleral route. However, drugs have occasional serious side effects, such as blurred vision, allergic reactions, headaches and potentially dangerous interactions with other drugs.
Surgical approaches for treating open-angle glaucoma consist of laser trabeculoplasty, trabeculectomy, and the implantation of aqueous shunts.
Trabeculectomy is a widely practiced surgery wherein microsurgical techniques are used to dissect the trabecular meshwork to allow more rapid aqueous outflow through the meshwork. Such procedures have proven to have limited long-term success. The dissection procedures often fail over time due to the body's wound healing response and resulting fibrosis that repairs and closes the dissected opening in the meshwork. After the dissections are healed up, the intraocular pressure again increases.
Implantable shunts and surgical methods are also known for providing a fluid path for aqueous humor to exit the anterior chamber of the eye to the sclera or a space beneath the conjunctiva.
Trabeculectomies and shunt surgeries and variations thereof have several disadvantages and moderate success rates. Such surgeries require significant surgical skills to create an incision through the full thickness of the sclera into the subconjunctival space. Further, the surgeries cause substantial trauma to the eye. The procedures are generally performed in an operating room and have a prolonged recovery time.